1. The Field Of The Invention
The present invention relates to a detector circuit for reflective mode optical mark readers and in particular to a detector circuit capable of operation in conjunction with a low level light source.
2. The Prior Art
It is well established that detecting diffusely reflected light through a suitable optical system works well for reading hand marked or printed data on paper. Many sources are in use for illumination in such readers. For example, incandescent lamps with optical filters and lenses supply relatively large amounts of light making detection problems rather easy.
The use of ordinary, readily available light emitting diodes for such readers is desirable from an economic standpoint. However, reflected light levels from LED'S are low and unusual circuit techniques are required to reliably detect contrast ratios resulting from data on the paper medium. Another advantage of using LED'S is that the type LED may be selected for various applications. No spectrum filters are necessary when the desired light wavelengths are produced directly by the light source. For example, visible red is ideal for detection of marks made by felt pens and ballpoint pens of the black, blue, and green variety. Pencil marks may also be detected, however, if detection of pencil marking only is desired, than infrared LEDs are the preferably light source. Red ink marks can be detected by using LEDs which emit visible green light as the light sources.